In a pressure vessel of a pressurized water reactor, there is provided a core barrel (also referred to as a “reactor core tank”) holding a reactor core including a fuel assembly. The core barrel has a function to support a weight of the fuel assembly and a function to position the fuel assembly.
In a conventional pressurized water reactor, the fuel assembly is placed on a lower core plate, a lower core support plate is provided under the lower core plate, and the lower core plate is attached to the lower core support plate through a lower core support column. With this configuration, the fuel assembly is supported by the lower core support plate constituting the core barrel.
The above-described lower core support column is fastened to the lower core plate and the lower core support plate with bolts or the like. With this arrangement, the fuel assembly may be damaged when the bolt is broken. In order to address this issue, a structure has been conceived in which a lower core support plate and a lower core plate are integrated so that the lower core support plate has a function of the lower core plate and in which a fuel assembly is placed on the lower core support plate.
FIG. 5 shows an example of a core barrel used in such a structure. A core barrel 50 shown in FIG. 5 includes a flange 51 to be attached to a reactor pressure vessel, an upper barrel 52 extending downward from the flange 51, a nozzle 53 provided on the upper barrel 52, an middle barrel 54 extending downward from the upper barrel 52, a lower barrel 55 extending downward from the middle barrel 54, the above-described lower core support plate 56 connected to the lower end part of the lower barrel 55. In the above members, the lower core support plate 56 has a placement surface 57 on which a fuel assembly is to be placed. The upper barrel 52, the middle barrel 54, and the lower barrel 55 are made, as shown in FIG. 6, by bended plate-shaped members in a cylindrical shape and by welding the bended members to one another, and on the respective curbed members, there are formed an upper barrel vertical weld line 58, an middle barrel vertical weld line 59, and a lower barrel vertical weld line 60. Further, as shown in FIG. 5, between the flange 51 and the upper barrel 52 is formed a flange circumferential weld line 61, between the upper barrel 52 and the middle barrel 54 is formed an upper barrel circumferential weld line 62, between the middle barrel 54 and the lower barrel 55 is formed an middle barrel circumferential weld line 63, and between the lower barrel 55 and the lower core support plate 56 is formed a lower core support plate circumferential weld line 64.
As described above, the core barrel is made in a welded can structure. However, if the weld line is faced to the fuel assembly, radiation emitted from the fuel assembly may cause, in some cases, stress corrosion cracking, thermal embrittlement, and irradiation embrittlement. For this reason, in the form in which a fuel assembly is supported through a lower core plate and a lower core support column by a lower core support plate, a core barrel manufactured by forging is known to reduce weld lines facing the fuel assembly.
In the process of manufacturing the core barrel shown in FIG. 5, first the lower core support plate 56 is machined to form the placement surface 57 and fuel alignment pin holes (not shown) in which fuel alignment pins for positioning the fuel assembly are to be inserted. Subsequently, the lower barrel 55 is welded to the lower core support plate 56. Then, the flange 51, the upper barrel 52, and the middle barrel 54 are welded to the lower barrel 55.
A high accuracy is generally required in a holding position of each fuel assembly in a reactor core. Therefore, it is desirable that a flatness of the placement surface 57 of the lower core support plate 56 and the positions or the shapes of the fuel alignment pin holes should have high accuracy.
However, lower core support plate circumferential weld line 64 is formed at a position close to the placement surface 57 of the lower core support plate 56. Therefore, due to influence of the welding when the lower core support plate circumferential weld line 64 is formed, it may happen that the accuracies of the flatness of the placement surface 57 or the fuel alignment pin holes which are already formed by machining cannot satisfy required values. In this case, it can be difficult to secure the accuracy of the position of each fuel assembly.